diff options
Diffstat (limited to 'kernel/rseq.c')
| -rw-r--r-- | kernel/rseq.c | 357 |
1 files changed, 357 insertions, 0 deletions
diff --git a/kernel/rseq.c b/kernel/rseq.c new file mode 100644 index 000000000000..ae306f90c514 --- /dev/null +++ b/kernel/rseq.c @@ -0,0 +1,357 @@ +// SPDX-License-Identifier: GPL-2.0+ +/* + * Restartable sequences system call + * + * Copyright (C) 2015, Google, Inc., + * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com> + * Copyright (C) 2015-2018, EfficiOS Inc., + * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> + */ + +#include <linux/sched.h> +#include <linux/uaccess.h> +#include <linux/syscalls.h> +#include <linux/rseq.h> +#include <linux/types.h> +#include <asm/ptrace.h> + +#define CREATE_TRACE_POINTS +#include <trace/events/rseq.h> + +#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \ + RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT) + +/* + * + * Restartable sequences are a lightweight interface that allows + * user-level code to be executed atomically relative to scheduler + * preemption and signal delivery. Typically used for implementing + * per-cpu operations. + * + * It allows user-space to perform update operations on per-cpu data + * without requiring heavy-weight atomic operations. + * + * Detailed algorithm of rseq user-space assembly sequences: + * + * init(rseq_cs) + * cpu = TLS->rseq::cpu_id_start + * [1] TLS->rseq::rseq_cs = rseq_cs + * [start_ip] ---------------------------- + * [2] if (cpu != TLS->rseq::cpu_id) + * goto abort_ip; + * [3] <last_instruction_in_cs> + * [post_commit_ip] ---------------------------- + * + * The address of jump target abort_ip must be outside the critical + * region, i.e.: + * + * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip] + * + * Steps [2]-[3] (inclusive) need to be a sequence of instructions in + * userspace that can handle being interrupted between any of those + * instructions, and then resumed to the abort_ip. + * + * 1. Userspace stores the address of the struct rseq_cs assembly + * block descriptor into the rseq_cs field of the registered + * struct rseq TLS area. This update is performed through a single + * store within the inline assembly instruction sequence. + * [start_ip] + * + * 2. Userspace tests to check whether the current cpu_id field match + * the cpu number loaded before start_ip, branching to abort_ip + * in case of a mismatch. + * + * If the sequence is preempted or interrupted by a signal + * at or after start_ip and before post_commit_ip, then the kernel + * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return + * ip to abort_ip before returning to user-space, so the preempted + * execution resumes at abort_ip. + * + * 3. Userspace critical section final instruction before + * post_commit_ip is the commit. The critical section is + * self-terminating. + * [post_commit_ip] + * + * 4. <success> + * + * On failure at [2], or if interrupted by preempt or signal delivery + * between [1] and [3]: + * + * [abort_ip] + * F1. <failure> + */ + +static int rseq_update_cpu_id(struct task_struct *t) +{ + u32 cpu_id = raw_smp_processor_id(); + + if (__put_user(cpu_id, &t->rseq->cpu_id_start)) + return -EFAULT; + if (__put_user(cpu_id, &t->rseq->cpu_id)) + return -EFAULT; + trace_rseq_update(t); + return 0; +} + +static int rseq_reset_rseq_cpu_id(struct task_struct *t) +{ + u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED; + + /* + * Reset cpu_id_start to its initial state (0). + */ + if (__put_user(cpu_id_start, &t->rseq->cpu_id_start)) + return -EFAULT; + /* + * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming + * in after unregistration can figure out that rseq needs to be + * registered again. + */ + if (__put_user(cpu_id, &t->rseq->cpu_id)) + return -EFAULT; + return 0; +} + +static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs) +{ + struct rseq_cs __user *urseq_cs; + unsigned long ptr; + u32 __user *usig; + u32 sig; + int ret; + + ret = __get_user(ptr, &t->rseq->rseq_cs); + if (ret) + return ret; + if (!ptr) { + memset(rseq_cs, 0, sizeof(*rseq_cs)); + return 0; + } + urseq_cs = (struct rseq_cs __user *)ptr; + if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs))) + return -EFAULT; + if (rseq_cs->version > 0) + return -EINVAL; + + /* Ensure that abort_ip is not in the critical section. */ + if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset) + return -EINVAL; + + usig = (u32 __user *)(rseq_cs->abort_ip - sizeof(u32)); + ret = get_user(sig, usig); + if (ret) + return ret; + + if (current->rseq_sig != sig) { + printk_ratelimited(KERN_WARNING + "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n", + sig, current->rseq_sig, current->pid, usig); + return -EPERM; + } + return 0; +} + +static int rseq_need_restart(struct task_struct *t, u32 cs_flags) +{ + u32 flags, event_mask; + int ret; + + /* Get thread flags. */ + ret = __get_user(flags, &t->rseq->flags); + if (ret) + return ret; + + /* Take critical section flags into account. */ + flags |= cs_flags; + + /* + * Restart on signal can only be inhibited when restart on + * preempt and restart on migrate are inhibited too. Otherwise, + * a preempted signal handler could fail to restart the prior + * execution context on sigreturn. + */ + if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) && + (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) != + RSEQ_CS_PREEMPT_MIGRATE_FLAGS)) + return -EINVAL; + + /* + * Load and clear event mask atomically with respect to + * scheduler preemption. + */ + preempt_disable(); + event_mask = t->rseq_event_mask; + t->rseq_event_mask = 0; + preempt_enable(); + + return !!(event_mask & ~flags); +} + +static int clear_rseq_cs(struct task_struct *t) +{ + /* + * The rseq_cs field is set to NULL on preemption or signal + * delivery on top of rseq assembly block, as well as on top + * of code outside of the rseq assembly block. This performs + * a lazy clear of the rseq_cs field. + * + * Set rseq_cs to NULL with single-copy atomicity. + */ + return __put_user(0UL, &t->rseq->rseq_cs); +} + +/* + * Unsigned comparison will be true when ip >= start_ip, and when + * ip < start_ip + post_commit_offset. + */ +static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs) +{ + return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset; +} + +static int rseq_ip_fixup(struct pt_regs *regs) +{ + unsigned long ip = instruction_pointer(regs); + struct task_struct *t = current; + struct rseq_cs rseq_cs; + int ret; + + ret = rseq_get_rseq_cs(t, &rseq_cs); + if (ret) + return ret; + + /* + * Handle potentially not being within a critical section. + * If not nested over a rseq critical section, restart is useless. + * Clear the rseq_cs pointer and return. + */ + if (!in_rseq_cs(ip, &rseq_cs)) + return clear_rseq_cs(t); + ret = rseq_need_restart(t, rseq_cs.flags); + if (ret <= 0) + return ret; + ret = clear_rseq_cs(t); + if (ret) + return ret; + trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, + rseq_cs.abort_ip); + instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); + return 0; +} + +/* + * This resume handler must always be executed between any of: + * - preemption, + * - signal delivery, + * and return to user-space. + * + * This is how we can ensure that the entire rseq critical section, + * consisting of both the C part and the assembly instruction sequence, + * will issue the commit instruction only if executed atomically with + * respect to other threads scheduled on the same CPU, and with respect + * to signal handlers. + */ +void __rseq_handle_notify_resume(struct pt_regs *regs) +{ + struct task_struct *t = current; + int ret; + + if (unlikely(t->flags & PF_EXITING)) + return; + if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq)))) + goto error; + ret = rseq_ip_fixup(regs); + if (unlikely(ret < 0)) + goto error; + if (unlikely(rseq_update_cpu_id(t))) + goto error; + return; + +error: + force_sig(SIGSEGV, t); +} + +#ifdef CONFIG_DEBUG_RSEQ + +/* + * Terminate the process if a syscall is issued within a restartable + * sequence. + */ +void rseq_syscall(struct pt_regs *regs) +{ + unsigned long ip = instruction_pointer(regs); + struct task_struct *t = current; + struct rseq_cs rseq_cs; + + if (!t->rseq) + return; + if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) || + rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) + force_sig(SIGSEGV, t); +} + +#endif + +/* + * sys_rseq - setup restartable sequences for caller thread. + */ +SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, + int, flags, u32, sig) +{ + int ret; + + if (flags & RSEQ_FLAG_UNREGISTER) { + /* Unregister rseq for current thread. */ + if (current->rseq != rseq || !current->rseq) + return -EINVAL; + if (current->rseq_len != rseq_len) + return -EINVAL; + if (current->rseq_sig != sig) + return -EPERM; + ret = rseq_reset_rseq_cpu_id(current); + if (ret) + return ret; + current->rseq = NULL; + current->rseq_len = 0; + current->rseq_sig = 0; + return 0; + } + + if (unlikely(flags)) + return -EINVAL; + + if (current->rseq) { + /* + * If rseq is already registered, check whether + * the provided address differs from the prior + * one. + */ + if (current->rseq != rseq || current->rseq_len != rseq_len) + return -EINVAL; + if (current->rseq_sig != sig) + return -EPERM; + /* Already registered. */ + return -EBUSY; + } + + /* + * If there was no rseq previously registered, + * ensure the provided rseq is properly aligned and valid. + */ + if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) || + rseq_len != sizeof(*rseq)) + return -EINVAL; + if (!access_ok(VERIFY_WRITE, rseq, rseq_len)) + return -EFAULT; + current->rseq = rseq; + current->rseq_len = rseq_len; + current->rseq_sig = sig; + /* + * If rseq was previously inactive, and has just been + * registered, ensure the cpu_id_start and cpu_id fields + * are updated before returning to user-space. + */ + rseq_set_notify_resume(current); + + return 0; +} |